Oncogene (2007) 26, 6697–6714 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW Epigenetic regulation of normal and malignant hematopoiesis KL Rice, I Hormaeche and JD Licht Division of Hematology/Oncology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA The molecular processes governing hematopoiesis involve the binding of sequence-specific transcription factors to the interplay between lineage-specific transcription factors target gene promoters and enhancers.These factors flag and a series of epigenetic tags, including DNA methyla- thoseregionsofthegenomedestinedtobetranscribedinto tion and covalent histone tail modifications, such as RNA, and work in part by recruitment of basal acetylation, methylation, phosphorylation, SUMOylation transcription factors and RNA polymerase II to target and ubiquitylation. These post-translational modifica- genes.Sequence-specific DNA-binding factors also recruit tions, which collectively constitute the ‘histone code’, are cofactors to gene regulatory regions, many of which are capable of affecting chromatin structure and gene part of multiprotein enzymatic complexes which facilitate transcription and are catalysed by opposing families of or inhibit gene transcription by modification of chromatin, enzymes, allowing the developmental potential of hema- the protein-bound state of DNA present in the cell topoietic stem cells to be dynamically regulated. The (Bottardi et al., 2007). Modulation of gene expression by essential role of these enzymes in regulating normal blood chromatin modification is termed ‘epigenetic’ regulation, development is highlighted by the finding that members and refers to stable and heritable changes in gene from all families of chromatin regulators are targets for expression that do not involve DNA sequence alterations. dysregulation in many hematological malignancies, and Such changes include DNA methylation, nucleosomal that patterns of histone modification are globally affected histone modifications, post-translational modifications in cancer as well as the regulatory regions of specific onco- and antisense miRNA silencing. genes and tumor suppressors. The discovery that these epigenetic marks can be reversed by compounds targeting aberrant transcription factor/co-activator/co-repressor interactions and histone-modifying activities, provides Nucleosomal histones: substrates for epigenetic the basis for an exciting field in which the epigenome of modification cancer cells may be manipulated with potential therapeutic benefits. The organization of DNA into higher order structures, Oncogene (2007) 26, 6697–6714; doi:10.1038/sj.onc.1210755 or nucleosomes, is a central component to epigenetic gene regulation.Each nucleosome, which represents the Keywords: hematopoiesis; epigenetics; histone code; basic repeating unit of chromatin, consists of 147 bp of methylation; leukemia; acetylation DNA wrapped around a core of eight histones including two molecules each of H2A, H2B, H3 and H4 (Luger et al., 1997). Individual nucleosomes are joined to each Overview other by the linker histone H1 and a short length of DNA (B200740 bp) to yield the 10 nm fiber, which Hematopoiesis is a dynamic process in which pluripo- may be further compacted into a helical structure tent, hematopoietic stem cells (HSCs) give rise to all the referred to as a 30 nm fiber via interactions between lineages of the blood, including T and B cells which the more variable, flexible histone tails, which protrude constitute the lymphoid lineage, and neutrophils, from the nucleosomal disk.The concept of a ‘histone eosinophils, basophils, monocytes, macrophages, mega- code’ was proposed following the discovery of specific karyocytes, platelets and erythrocytes which comprise post-translational covalent modifications of these his- the myeloid lineage (Zhu and Emerson, 2002).This tone tails by acetylation, methylation, phosphorylation, process involves the coordination of signal transduction glycosylation, SUMOylation and ubiquitylation.Such pathways, which are responsive to extracellular stimuli, modifications act in a concerted manner to induce and transcriptional networks affecting gene expression, structural changes in the chromatin fiber and to regulate such that the ultimate fate of the active HSC pool is the accessibility of transcription factors to gene regula- linked to the functional needs of the organism.The tory sequences, ADP ribosylate affecting gene expres- regulation of gene transcription is critically mediated by sion (Jenuwein and Allis, 2001).There are a vast number of potential combinations of chromatin modifications that can be displayed by histones but several general- Correspondence: Dr JD Licht, Division of Hematology/Oncology, Feinberg School of Medicine, Northwestern University, Lurie 5-123, izations can be made.Transcribed genes may be present 303 East Superior Street, Chicago, IL 60611, USA. in nucleosome-free regions that are highly accessible to E-mail: [email protected] transcription factors, or in regions of chromatin that Epigenetic regulation of hematopoiesis KL Rice et al 6698 Figure 1 Sequence-specific transcription factors act as docking molecules for the recruitment of DNA and histone-modifying activities to target gene promoters.Active transcription is associated with hyperacetylation and methylation of H3K4, H3K79 and H3K36 residues in promoter regions, whereas gene repression is associated with DNA methylation, hypoacetylation and methylation of H3K9, H3K27 and H4K20 residues.These modifications are mediated by chromatin-modifying enzymes including DNA methyltransferases (DNMTs), histone acetyltransferases (HATs)/histone deacetylases (HDACs), histone methyltransferases (HMTs) and histone demethylases (HDMs). tend to be hyperacetylated through the action of histone domains.A number of DNA-binding domains acetyltransferases (HATs).By contrast, heterochromatic have been characterized including the homeodomain, regions, generally silent in terms of gene expression, tend zinc-finger domain, leucine zipper domain, winged helix, to be hypoacetylated through the action of histone erythroblast transformation-specific domain (ETS) and deacetylases (HDACs) and methylated on cytosine- helix–loop–helix domains.In the past, the determination phosphate-guanine (CpG) dinucleotides by DNA methy- of targets of transcription factors was based on a ltransferases (DNMTs).Acetylation of histones can candidate gene approach.With the advent of technol- change the physicochemical properties of these proteins, ogies such as chromatin immunoprecipitation-promoter interfering with the electrostatic attraction between tiling arrays (ChIP–CHIP) and protein-binding micro- positively charged histones and negatively charged arrays, the ability to probe transcription factor-binding DNA.Furthermore, specific histone tail modifications sites on a comprehensive, genome-wide level is now offer binding sites for the recruitment of other chromatin possible (Mukherjee et al., 2004; Oberley et al., 2004). modification machinery.For example, specific histone tail These studies now allow the identification of canonical residue methylation events may be associated with and novel binding motifs within promoter, enhancer and gene activation and others with gene repression transcribed regions of target genes (O’Geen et al., 2007). (Figure 1). ChIP–CHIP experiments also reveal the presence of transcription-priming mechanisms, which involve pre- assembly of transcriptional machinery at promoters in preparation for mitogenic stimulation, and binding site Sequence-specific transcription factors in hematopoiesis selection mechanisms, which reveal that transcription factor binding is not only dependent on sequence During hematopoiesis, the controlled expression of recognition but also on chromatin structure that is lineage-specific genes is crucial for proliferation and associated with specific epigenetic marks found in differentiation cues.Many transcription factors are ‘euchromatic islands’.For example, transcriptional absolutely essential for the development of a hemato- regulation by Myc involves recognition of E-boxes poietic lineage, for example GATA-1 is required for the within the context of trimethylated H3 lysine 4 erythroid and megakaryocytic lineage, while PU.1 is (H3K4) and H3 acetylation and typically correlates required for myeloid development.The retinoic acid with preassembled RNA Pol II (Guccione et al., 2006). receptor (RAR), which is required for neither lineage clearly plays a modulatory role in the production of blood cells.The ability of such factors to influence cell Recruit either co-activators or co-repressors to the fate is related to the ability of transcription factors to: regulatory regions of genes via protein interaction domains Recognize specific DNA sequences via DNA-binding The ability of transcription factors to affect gene domain transcription is dependent on the specific association Transcription factors are modular with generally of activator or repressor regions of transcription factors distinct DNA-binding and transcriptional effector with co-activators or repressors.These cofactors may Oncogene Epigenetic regulation of hematopoiesis KL Rice et al 6699 serve to scaffold basal transcriptional machinery to malignancy.In hematological malignancies, recurring target genes, recruit chromatin-modifying enzymes (see chromosomal translocations may lead to the formation below) or to covalently modify transcription factors of novel fusion proteins or